K. C. Cole

Orientation and structure of drawn poly(ethylene terephthalate)

Abstract Drawing of poly(ethylene terephthalate) (PET) films was performed from the amorphous state at different drawing rates and at 80°C. Crystallinity of the films was determined by thermal analysis and orientation of the different phases was determined by birefringence, Fourier transform infra-red ( FT i.r.) specular reflection and wide angle X-ray diffraction. The mechanical properties of the oriented films were studied by dynamic mechanical analysis. FT i.r. and X-ray results showed large orientation of the different phases ( trans conformers and crystalline phase), in contrast with refractive index measurements which showed unusually low birefringence results. The observed orientation was particularly high for draw ratios (λ) higher than 3. The orientation contribution of the non-crystalline phase was determined by combining FT i.r. and X-ray results. The trans -conformer contribution to the non-crystalline phase is shown to be very high for the high λ for which crystalline orientation was observed. A structural model consisting of a crystalline phase, a mesomorphic amorphous phase (constituted of trans conformers) and a purely amorphous phase (constituted solely of gauche conformers) is a possible representation of the observed results. Finally, the tensile modulus increased continuously with the overall orientation.

Characterization of Surface Orientation in Poly(Ethylene Terephthalate) by Front-Surface Reflection Infrared Spectroscopy

The use of front-surface specular reflection FT-IR spectroscopy to characterize surface orientation in thick samples of poly(ethylene terephthalate) (PET) has been investigated. It has been shown that, even for samples whose surface uniformity is less than perfect, absorption index spectra of excellent quality are obtained from the Kramers-Kronig transformation. These spectra provide detailed information, both qualitative and quantitative, on the molecular conformation and orientation of the polymer. Of particular interest are the strongly absorbing bands such as the carbonyl and ester peaks, which are generally saturated in transmission spectra. The reflection technique opens up the possibility of obtaining new information on conformational changes and orientation in PET through a detailed study of these peaks.

Amorphous orientation of poly(ethylene terephthalate) by X-ray diffraction in combination with Fourier transform infra-red spectroscopy

Drawing of poly(ethylene terephthalate) (PET) films was performed from the amorphous state using different drawing rates at a temperature of 80°C. The crystallinity of the films was determined by thermal analysis and orientation of the different phases by specular reflection Fourier transform infra-red (FTi.r.) spectroscopy and wide-angle X-ray diffraction. The FTi.r. spectroscopic and X-ray results showed a significant orientation of the different molecular species (trans- and gauche-conformers and crystalline phase), particularly for draw ratios (λ) greater than 3. Determination of the amorphous orientation from X-ray diffraction of the amorphous phase of PET is shown to be possible. A combination of the FTi.r. results with those obtained for the X-ray orientation of the crystalline phase allowed an independent determination of the amorphous orientation. A good agreement was observed with the corrected X-ray amorphous orientation results for draw ratios greater than 3. Below this draw ratio, low orientation and crystallinity, combined with the inherent limitation of the X-ray technique and crystal imperfections, induced large discrepancies between the two results. It is also shown that the trans-conformer orientation increases steadily with draw ratio from the onset of draw and saturates rapidly, whereas that of the gauche-conformer is negligible for all draw ratios.

Orientation and conformation in poly(ethylene terephthalate) with low draw ratios as characterized by specular reflection infra-red spectroscopy

A quantitative treatment of the specular reflection spectra obtained from the surface of uniaxially drawn poly(ethylene terephthalate) samples has been performed. A procedure for correcting for the effect of surface irregularities is presented, and an overall orientation function based on the orientation and content of trans conformers is calculated. The results are correlated with mechanical modulus and crystallinity values. In addition, an unconventional dichroic ratio parameter based on a combination of two major bands is proposed. Results obtained from the Kramers-Kronig analysis and directly from the reflection spectra are discussed. Both are compared with the overall orientation function obtained before.

Fourier transform infrared spectroscopic study of thermal degradation in films of poly(etheretherketone)

Abstract Fourier transform infrared (FT-IR) spectroscopy was used to study the reactions which occur upon thermal degradation of films of poly(etheretherketone), or PEEK. Samples were exposed to temperatures in the range 400–485°C in both air and nitrogen atmospheres, and spectra were measured in both transmission and attenuated total reflection (ATR). The thermal degradation produces new carbonyl species, and the rate of growth of their IR peaks was used to determine activation energies for the reactions which produce them. In an inert atmosphere, the degradation involves a pyrolytic-type mechanism which produces a new carbonyl species absorbing in the IR at 1711 cm −1 , possibly a fluorenone-type structure. The activation energy for this process is 236 kJ mol −1 . In an oxidizing atmosphere, the same species is produced but at a faster rate, and the apparent activation energy is 211 kJ mol −1 . In addition there is a second mechanism which requires the participation of oxygen and produces a species absorbing at 1739 cm −1 , possibly ester groups. The activation energy for this reaction is 116 kJ mol −1 . Because of the time required for oxygen to diffuse into the polymer, oxidative degradation is more pronounced at the surface, especially at higher temperatures. Thus ATR gives a more reliable value of the activation energies than transmission measurements on the bulk film.

New insights into the development of ordered structure in poly(ethylene terephthalate), II Results from transmission infrared spectroscopy of thin films

To complement our earlier work involving external reflection infrared spectroscopy of PET, a set of thin PET films with different crystalline structures (amorphous, strain-induced crystallinity, thermally induced crystallinity) was prepared and analyzed in detail in the transmission mode. In analogy with the reflection work, factor analysis was used to generate three basis spectra corresponding to three distinct structures: G = an arrangement involving gauche glycol conformers and disordered terephthalate groups; TX = an arrangement involving trans glycol conformers and disordered terephthalate groups (probably a mixture of cis and trans conformers); and TC = the all-trans arrangement found in the true crystalline phase. The TX structure is believed to play an important role in the widely reported intermediate phase of PET, which is particularly significant in cold-drawn samples. The transmission spectra confirm the validity of the reflection spectra and also provide more detailed information. In addition, analysis of two biaxially oriented industrial films has provided further information on the geometry of certain vibrational modes. Overall, the results lead to a better understanding of the complex crystalline structure of PET and its relationship to the infrared spectrum.

NEW APPROACH TO QUANTITATIVE ANALYSIS OF TWO-COMPONENT POLYMER SYSTEMS BY INFRARED SPECTROSCOPY

A new treatment is proposed for quantitative analysis of two-component polymer systems by infrared spectroscopy. Like much previous work, it is based on a ratio involving two peaks in the same spectrum. The relationship between such a ratio and the concentration of a given polymer is inherently nonlinear. It is shown that this nonlinearity can be well described by a simple equation derived from the laws of optical transmission. This equation has the form χ1 = m1 + m2R/(1 + m3R), where χ1 is the weight fraction of polymer 1, the mi are adjustable coefficients, and the ratio R is equal to Aa/(Aa + Ab). The quantities Aa and Ab are the absorbances (peak heights or areas) at two frequencies a and b of which the first is associated mainly with polymer 1 and the second with polymer 2. This equation has been applied to various peak combinations in spectra of miscible blends of poly(phenylene ether) with polystyrene (both mid-IR and near-IR data) and immiscible blends of polypropylene with polyethylene (mid-IR data). It is shown that the equation is valid in all cases, covering the full concentration range from 0 to 100% even when the peaks used for the analysis involve absorption by both polymers. It is therefore believed to be of broad general usefulness for the analysis of polymer blends and copolymers.

Comparison of Infrared Spectroscopic Methods for the Quantitative Analysis of Epoxy Resins Used in Carbon-Epoxy Composite Materials

Three different methods of obtaining the infrared spectrum of carbon-epoxy prepreg for quality control purposes are critically evaluated with respect to their suitability for quantitative analysis. They are: transmission using a cast film of resin, attenuated total reflection (ATR) using prepreg, and diffuse reflection (DR) using prepreg. The cast film method is subject to significant error resulting from nonuniform film thickness; careful sample preparation is necessary in order to obtain acceptable results. The ATR method is faster and easier to use than the transmission method and gives more reproducible results; the use of a 45° germanium prism is recommended. The DR method is not suitable for quantitative work because of complications arising from front-surface reflection effects; it can be very useful, however, as a qualitative method.

Quantitative Analysis of Monomer Composition in Ethylene-Propylene Block Copolymers by FT-IR Spectroscopy:

The quantitative analysis of ethylene (or propylene) in the commercially important ethylene-propylene (EP) copolymers has been the subject of considerable research. Although NMR is the most useful method, it does present certain problems such as solubility and cost. IR is therefore still preferred from the industrial point of view. In IR, most of the work has been done on high-ethylene-content EP copolymers, with the use of conventional grating spectrophotometers. The two IR regions usually employed are at 1460–1380 and 1160–720 cm−1, and these have been studied for blends of homopolymers ranging from 0–100 wt % polyethylene (PE) and for random copolymers containing 42–66.7 wt % PE. The analyses are based on measurements of the ratios of absorbances at 1380–1460 and 1160–720 cm−1, as well as the ratios of peak areas for the two IR regions. Difficulties arise when one is attempting to accurately measure absorbance or peak area ratios at the two extremes of the concentration ranges.

Effect of copper on the curing and structure of a DICY-containing epoxy composite system†

The curing and structure of an epoxy system containing dicyandiamide (DICY) as hardener were studied as a function of temperature and the presence or absence of copper with the use of differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy in photoacoustic mode (FTIR-PAS). Spectroscopic analysis of specimens taken from the DSC helped to clarify the reaction mechanism in terms of the different schemes that have been proposed. The initial stages, corresponding to the first peak in the DSC exotherm, involve the usual epoxide-amine reactions closely followed by a reaction between DICY nitrile groups and hydroxyl groups to form structures containing iminoether and urea groups. These reactions are slightly retarded in the presence of copper. At higher temperatures, corresponding to the second peak in the exotherm, these structures are transformed into others believed to contain urethane ester groups. This reaction, which may be considered to constitute a form of degradation, is significantly accelerated in the presence of copper. The effect is particularly large around 180°C, a temperature commonly used to cure such systems, so the results have important practical implications, for example, in the lamination of circuit boards.